A finite impulse response filter, including a plurality of taps arranged to receive and process a sequence of input data samples so as to generate a filter output. Each tap consists of a multiplier operating in one's complement arithmetic, the multiplier being coupled to multiply a respective input sample from the sequence by a respective equalization coefficient, and an adder, which sums an output from the multiplier. The taps are arranged in sequence so that the input sample to each of the taps, except to a first tap in the sequence, is delayed relative to a preceding tap in the sequence. The filter also includes an adjustment-accumulator coupled to receive the filter output and responsive thereto to generate an adjustment that is adapted to correct the filter output to a twos complement result, and an adjustment-adder which sums the adjustment and the filter output to generate a final output.
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6. A method for filtering a signal, comprising:
receiving and processing a sequence of input data samples in a plurality of taps so as to generate a filter output, each tap comprising:
a multiplier operating in ones complement arithmetic, the multiplier being coupled to multiply a respective input sample from the sequence by a respective equalization coefficient; and
an adder, which sums an output from the multiplier, the taps being arranged in sequence so that the input sample to each of the taps, except to a first tap in the sequence, is delayed relative to a preceding tap in the sequence;
receiving the filter output and responsive thereto generating an adjustment that is adapted to correct the filter output to a twos complement result; and
summing the adjustment and the filter output to generate a final output.
1. A finite impulse response filter, comprising:
a plurality of taps arranged to receive and process a sequence of input data samples so as to generate a filter output, each tap comprising:
a multiplier operating in ones complement arithmetic, the multiplier being coupled to multiply a respective input sample from the sequence by a respective equalization coefficient; and
an adder, which sums an output from the multiplier, the taps being arranged in sequence so that the input sample to each of the taps, except to a first tap in the sequence, is delayed relative to a preceding tap in the sequence;
an adjustment-accumulator coupled to receive the filter output and responsive thereto to generate an adjustment that is adapted to correct the filter output to a twos complement result; and
an adjustment-adder which sums the adjustment and the filter output to generate a final output.
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This application claims the benefit of U.S. Provisional Patent Application 60/341,526, filed Dec. 17, 2001, which is incorporated herein by reference.
The present invention relates generally to data communication, and specifically to communicating data using multiple physical lines.
As communication speeds have increased, the demands to transmit signals over existing infrastructures have become significantly harder to meet. Four twisted pair Ethernet cabling, originally conceived for conveying signals at 1 or 10 Mb/s, is now required to convey signals at rates of the order of 1 Gb/s. Inter alia, the increased throughput leads to increased processing requirements for received signals as well as increased impairment of the received signals.
An IEEE standard 802.3ab, published by the Institute of Electronic and Electrical Engineers, New York, N.Y., describes an Ethernet protocol wherein data may be transmitted as five-level pulse amplitude modulation (PAM-5) signals over category-5 cables, comprising four pairs of twisted wires. The data may be transmitted in a full-duplex mode at rates of the order of 1 Gb/s. As in most data transmission systems, signal degradation along a transmission path means that signal recovery becomes increasingly more difficult as the path length increases, and/or as the rate of transmission increases. In particular, recovering the clocks for such degraded signals is a significant problem as signal frequencies increase, both because of the increased degradation of the signals and also because of the reduced time available for processing the signals.
In a paper by Mueller and Muller, “Timing recovery in digital synchronous data receivers,” IEEE Transactions on Communications, pp 516–531, Vol. 24, May 1976, the authors propose a timing recovery algorithm. The paper is accepted in the art as the basis for timing recovery algorithms, and relies on selecting a timing function of a best sampling point. The phase of the sampling point is then adjusted until its timing function is zero.
U.S. Pat. No. 6,192,072, to Azadet et al., whose disclosure is incorporated herein by reference, describes a parallel processing decision feedback equalizer (DFE) which may be applied to recovering the clocks from IEEE 802.3ab signals transmitted on four pairs of wires. The method relies on multiple clock domains, respective clock recovery being performed on each pair of wires.
With the increase of data speeds, receivers operating at the increased frequencies increasingly suffer from extraneous noise introduced into the data transmission lines. The receivers require filters to reduce the effects of such noise. However, filters known in the art occupy considerable chip area, and also consume significant amounts of power.
The present invention seeks to provide a finite impulse response filter which performs internal operations within the filter using ones complement arithmetic. One or more such filters may be advantageously incorporated into an echo canceller and also into near end cross-talk (NEXT) cancellers in a data receiver.
In preferred embodiments of the present invention, a finite impulse response (FIR) filter comprises a plurality of adaptive taps, at least some of which comprise a delay, an adder, and a multiplier. Operations performed by each multiplier of the filter taps are performed using ones complement arithmetic. The filter further comprises an adjustment-accumulator which is coupled to a final tap of the FIR filter, and which receives an output of the final tap. The adjustment-accumulator acts to adjust the final tap output so as to generate a final output from the filter equivalent to a twos complement output. By using ones complement arithmetic, chip area usage and power needed for data toggling are both reduced, compared to systems using other forms of arithmetic.
Preferably, the taps of the FIR filter are arranged in a direct-form architecture, a transpose-form architecture, or a combination of the two forms of architecture as a hybrid, in order to reduce the total number of elements in the filter.
In some preferred embodiments of the present invention, data is input to the filter on multiple levels, preferably five levels, and adaptation is preferably performed on a subset of the data, the data subset corresponding to data received on a corresponding subset of the levels. Preferably, the subset of the levels comprises levels—in the case of five levels the two most extreme values—having the highest energies of data transfer. Performing adaptation on data from a subset of the multiple levels reduces power consumed for the adaptation. By choosing the subset so that high data energies are used for the adaptation, the efficiency of adaptation is substantially unaffected, despite operating only on the data subset and not on all the data.
The times at which the adaptation occurs may be decimated, in which case the decimation is most preferably performed in a substantially random fashion. The decimation is randomized by selecting one of the levels at which the data is input to the filter. A modulo count of the data at the selected level is maintained, and at a predetermined value of the count, adaptation is performed on a tap of the filter, preferably the first tap, based on the data entering the filter at the time of the count. Most preferably, as this data traverses the filter, adaptation is performed on subsequent taps of the filter using this data. Furthermore, the filter preferably comprises a monitor to ensure that the rate at which adaptation is performed does not fall below a predefined minimum frequency. Counting incoming data at a specific level, and applying the count as described above, enables adaptation to be randomized over frequencies and over different taps of the filter. If randomization is not performed, e.g., if the adaptation is at a low fixed frequency, the adaptation does not function well.
Transferring the multi-level data between elements of the filter is most preferably performed by encoding each level into a unique string of multiple bits, each string preferably comprising three bits, so that there is a one-to-one mapping between levels and strings. As required during operation of the filter, each string is decoded to recover the level associated with the string. When data changes, from a first level encoded as a first string to a second level encoded as a second string, there is switching activity in the filter caused by toggling between the bits of the two strings. A switching activity value for the mapping may be calculated as a sum of toggles between all the possible different levels. Preferably, the encoding is selected so as to reduce the switching activity to a minimum value. Reducing the switching activity caused by data level changes leads to a corresponding reduction in power consumed by the filter.
An echo canceller in a data receiver according to an embodiment of the present invention preferably comprises a first FIR filter which acts as a near echo canceller. The canceller also incorporates one or more subsequent FIR filters, preferably two filters, which act as round trip delay (RTD) filters, and which are separated from the near echo filter by a variable delay line. The delay line introduces a delay between output of transmit data from the near echo filter and input of the data to the RTD filters. The canceller also comprises a delay line controller which measures energy absorbed by the RTD filters, and which adjusts the delay responsive to the measured energy. The delay is preferably adjusted to maximize the energy absorbed. By means of the variable delay, the number of taps required in the RTD filters may be reduced, compared to systems which utilize a full echo canceller. Furthermore, when two RTD filters are used, one of the RTD filters may be powered down, depending on the energy measured by the delay line controller, reducing power consumption with substantially no reduction in energy absorbed by the canceller.
There is therefore provided, according to a preferred embodiment of the present invention, a finite impulse response filter, including:
a plurality of taps arranged to receive and process a sequence of input data samples so as to generate a filter output, each tap including:
a multiplier operating in ones complement arithmetic, the multiplier being coupled to multiply a respective input sample from the sequence by a respective equalization coefficient; and
an adder, which sums an output from the multiplier, the taps being arranged in sequence so that the input sample to each of the taps, except to a first tap in the sequence, is delayed relative to a preceding tap in the sequence;
an adjustment-accumulator coupled to receive the filter output and responsive thereto to generate an adjustment that is adapted to correct the filter output to a twos complement result; and
an adjustment-adder which sums the adjustment and the filter output to generate a final output.
Preferably, each of the respective equalization coefficients are adaptive in response to an error signal input to the filter.
The filter preferably further includes a sign-determining component which determines a sign and an absolute value of each of the input data samples, and preferably, if the sign is negative the adjustment-accumulator adds the absolute value to the filter output, and if the sign is positive, the adjustment-accumulator subtracts the absolute value from the filter output.
Preferably, the plurality of taps are arranged in an architecture chosen from a direct-form architecture, a transpose-form architecture, and a hybrid-form architecture.
There is further provided, according to a preferred embodiment of the present invention, a method for performing adaptation on taps comprised in a finite impulse response filter, including:
receiving input-data at a plurality of levels at the filter;
selecting analysis-data from the input-data, the analysis-data comprising a subset of the plurality of levels; and
adapting coefficients of the taps responsive to the analysis-data.
Preferably, the plurality of levels includes five levels, and the subset includes a highest and a lowest of the five levels, and the five levels consists of a set of values +2, +1, 0, −1, and −2.
There is further provided, according to a preferred embodiment of the present invention, a method for performing adaptation decimation on taps comprised in a finite impulse response filter, including:
receiving input-data at a plurality of levels at the filter;
selecting analysis-data from the input-data, the analysis-data comprising a subset of the plurality of levels;
performing a count of the analysis-data; and
at a predetermined value of the count, adapting coefficients of the taps responsive to the input-data.
Preferably, performing the count includes counting cyclically.
Preferably, the plurality of levels includes five levels, and the subset consists of a highest and a lowest of the five levels.
The method preferably includes monitoring a time at which adapting the coefficients is performed, and performing an adaptation responsive to the time.
There is further provided, according to a preferred embodiment of the present invention, a method for coding data received in a finite impulse response filter, including:
receiving the data at the filter at a plurality of different levels;
generating one or more encodings, each encoding mapping each of the different levels to a respective one of a plurality of unique binary strings, based on a one-to-one relationship between the different levels and the unique binary strings;
determining, for each of the one or more encodings, a respective switching activity value caused by toggling between the unique binary strings responsive to transitions between the plurality of different levels in the received data; and
selecting an encoding-for-coding-the-data from the one or more encodings responsive to the respective switching activity values.
Preferably, the encoding includes a set defined by a relationship {(level, string)}={(+2,010), (+1,001), (0,000), (−1,100), (+1,110)}.
There is further provided, according to a preferred embodiment of the present invention, data filtering apparatus, including:
a finite impulse response filter which receives data and which performs a preliminary filtration thereupon to cancel an echo present in the data and to generate preliminary output data;
a delay line which receives the preliminary output data and which is adapted to insert a delay into the preliminary output data to generate delayed data;
at least one round trip delay (RTD) filter which is adapted to receive the delayed data and to perform a further filtration thereupon to cancel a round trip delay signal present in the data and to generate further output data; and
a delay line controller which measures delayed data energy absorbed by the at least one RTD filter responsive to receiving the delayed data and which sets the delay responsive to the delayed data energy absorbed.
Preferably, the echo includes a near-end echo remaining in the data, the near-end echo being generated by a transmitter coupled to the apparatus.
Preferably, the delay line controller is adapted to perform sequential adjustments to the delay, and to measure the delayed data after each adjustment.
Further preferably, the at least one RTD filter includes a plurality of tap coefficients, and the delayed data energy absorbed is a function of a sum of the plurality of the tap coefficients.
Preferably, the at least one RTD filter includes a first and a second RTD filter, the delay includes a first delay applied to the first RTD filter and a second delay applied to the second RTD filter, the delayed data energy absorbed includes a first-RTD-filter-delayed-data-energy-absorbed and a second-RTD-filter-delayed-data-energy-absorbed, and the delay line controller sets the first delay responsive to the first-RTD-filter-delayed-data-energy-absorbed and the second delay responsive to the second-RTD-filter-delayed-data-energy-absorbed.
Preferably, the first RTD filter includes a first set of taps and the second RTD filter includes a second set of taps, and the delay line controller is adapted to adjust the first delay and the second delay so that none of the first set of taps and the second set of taps have equal delays.
Preferably, the delay line controller is adapted to power down the first RTD filter responsive to the first-RTD-filter-delayed-data-energy-absorbed and the second-RTD-filter-delayed-data-energy-absorbed.
There is further provided, according to a preferred embodiment of the present invention, a method for filtering a signal, including:
receiving and processing a sequence of input data samples in a plurality of taps so as to generate a filter output, each tap consisting of:
a multiplier operating in ones complement arithmetic, the multiplier being coupled to multiply a respective input sample from the sequence by a respective equalization coefficient; and
an adder, which sums an output from the multiplier, the taps being arranged in sequence so that the input sample to each of the taps, except to a first tap in the sequence, is delayed relative to a preceding tap in the sequence;
receiving the filter output and responsive thereto generating an adjustment that is adapted to correct the filter output to a twos complement result; and
summing the adjustment and the filter output to generate a final output.
Preferably, each of the respective equalization coefficients is adaptive in response to an error signal input to the filter.
The method preferably further includes determining a sign and an absolute value of each of the input data samples, and adding the absolute value to the filter output if the sign is negative, and subtracting the absolute value from the filter output if the sign is positive.
Preferably, the method includes arranging the plurality of taps in an architecture chosen from a direct-form architecture, a transpose-form architecture, and a hybrid-form architecture.
There is further provided, according to a preferred embodiment of the present invention, apparatus for performing adaptation on taps comprised in a finite impulse response filter, including:
a processor which is adapted to:
receive input-data at a plurality of levels at the filter,
select analysis-data from the input-data, the analysis-data comprising a subset of the plurality of levels, and
adapt coefficients of the taps responsive to the analysis-data.
Preferably, the plurality of levels includes five levels, and the subset includes a highest and a lowest of the five levels, and the five levels consist of a set of values +2, +1, 0, −1, and −2.
There is further provided, according to a preferred embodiment of the present invention, apparatus for performing adaptation decimation, including:
a finite impulse response filter, consisting of taps, that receives input-data at a plurality of levels;
a selector that selects analysis-data from the input-data, the analysis-data comprising a subset of the plurality of levels;
a counter that performs a count of the analysis-data and that, at a predetermined value of the count, adapts coefficients of the taps responsive to the input-data.
Preferably, the counter counts cyclically.
Preferably the plurality of levels includes five levels, and the subset includes a highest and a lowest of the five levels.
Preferably, the counter is adapted to monitor a time at which the coefficients are adapted, and to perform an adaptation responsive to the time.
There is further provided, according to a preferred embodiment of the present invention, apparatus for coding data received in a finite impulse response filter, including:
a processor which is adapted to:
receive the data at the filter at a plurality of different levels,
generate one or more encodings, each encoding mapping each of the different levels to a respective one of a plurality of unique binary strings, based on a one-to-one relationship between the different levels and the unique binary strings,
determine, for each of the one or more encodings, a respective switching activity value caused by toggling between the unique binary strings responsive to transitions between the plurality of different levels in the received data, and
select an encoding-for-coding-the-data from the one or more encodings responsive to the respective switching activity values.
Preferably, the encoding includes a set defined by a relationship {(level, string)}={(+2,010), (+1,001), (0,000), (−1,100), (+1,110)}.
There is further provided, according to a preferred embodiment of the present invention, a method for filtering data, including:
receiving the data in a finite impulse response filter;
performing a preliminary filtration on the data in the finite impulse response filter so as to cancel an echo present in the data and to generate preliminary output data;
inserting a delay into the preliminary output data to generate delayed data;
receiving the delayed data in at least one round trip delay (RTD) filter;
performing a further filtration on the delayed data in the at least one RTD filter so as to cancel a round trip delay signal present in the data and to generate further output data;
measuring delayed data energy absorbed by the at least one RTD filter responsive to receiving the delayed data; and
setting the delay responsive to the delayed data energy absorbed.
Preferably, the echo includes a near-end echo remaining in the data, the near-end echo being generated by a transmitter coupled to the filter.
The method preferably includes performing sequential adjustments to the delay, and measuring the delayed data after each adjustment.
Preferably, the at least one RTD filter includes a plurality of tap coefficients, and the delayed data energy absorbed is a function of a sum of the plurality of tap coefficients.
Preferably, the at least one RTD filter includes a first and a second RTD filter, the delay includes a first delay applied to the first RTD filter and a second delay applied to the second RTD filter, the delayed data energy absorbed includes a first-RTD-filter-delayed-data-energy-absorbed and a second-RTD-filter-delayed-data-energy-absorbed, and setting the delay includes setting the first delay responsive to the first-RTD-filter-delayed-data-energy-absorbed and setting the second delay responsive to the second-RTD-filter-delayed-data-energy-absorbed.
Preferably, the first RTD filter includes a first set of taps and the second RTD filter includes a second set of taps, and setting the delay includes adjusting the first delay and the second delay so that none of the first set of taps and the second set of taps have equal delays.
The method preferably also includes powering down the first RTD filter responsive to the first-RTD-filter-delayed-data-energy-absorbed and the second-RTD-filter-delayed-data-energy-absorbed.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which:
Reference is now made to
Transceiver 20 communicates via line 22 with a remote transceiver 33, which is coupled to the line and which operates according to the Ethernet protocol. Transceiver 33 is preferably implemented substantially as described herein for transceiver 20. Alternatively, transceiver 33 comprises any transceiver which operates according to the Ethernet protocol. As described in the protocol, an initialization phase, when transceivers 20 and 33 are to communicate, comprises an auto-negotiation stage followed by a start-up stage which may include an equalization stage. During these stages, transceivers 20 and 33 agree on which transceiver is to act as a master in the ensuing communication, and which transceiver is to act as a slave. The transceiver which is assigned to be the master starts to transmit immediately. The slave starts to transmit after completing the equalization stage.
Transceiver 20 consists of a substantially analog section 21, and a substantially digital section 23. Analog section 21 comprises four substantially similar analog transmitter modules 40A, 40B, 40C, and 40D, each of which transmits data to one of the conductor pairs in line 22. Transmitter modules 40A, 40B, 40C, and 40D are also collectively referred to herein as transmitter module 40. Each module 40 receives digital data from a physical coding sub-layer (PCS) 32, and converts the digital data to analog two-bit symbols using a digital to analog converter (DAC) 29 present in each module. The conversion is performed every 8 ns, according to a single 125 MHz clock signal received from a phase locked loop clock generator (PLL) 38, the single clock signal providing a common shared clock domain within which elements of transceiver 20 operate.
Analog section 21 also comprises four substantially similar analog receiver modules 26A, 26B, 26C, and 26D, each of which receives data from one of the conductor pairs in line 22. Receiver modules 26A, 26B, 26C, and 26D are also collectively referred to herein as receiver module 26. In order to reduce interference between each transmitter module 40 and its corresponding receiver module 26, each transmitter module includes a programmable hybrid circuit 31. The hybrid circuit conveys a delayed portion of a transmitted signal from each transmitter module 40 to the respective receiver module 26, the receiver module using the delayed portion to reduce echo in the received signal. Such methods for reducing echo are known in the art.
A variable gain amplifier (VGA) 25 in each receiver module 26 receives the signal from its respective conducting pair, and adjusts the signal level to a value suited to a respective following analog to digital converter (ADC) 27. Each ADC 27, preferably a 7 bit flash ADC, receives the 125 MHz clock generated by PLL 38 and performs sampling at 4 ns intervals, so generating two samples for each clock period of 8 ns. The two samples are transferred, in parallel, for processing in digital section 23, as is described below.
It will be appreciated that by sampling each ADC 27 with the same clock signal, no fluctuation between sampling times of analog receiver modules 26 occurs, not even short-term fluctuations. In contrast, receivers using separate clocks to sample each receiver will of necessity experience at least short-term fluctuations between clock signals. Because of the absence of fluctuations between sampling times, there is substantially no interference between conducting pairs in line 22, and near end cross talk (NEXT) cancellation is thus significantly improved, especially at clock frequencies.
Raw digitized samples produced by each ADC 27 in receiver modules 26A, 26B, 26C, and 26D are transferred to respective sub-receiver modules 28A, 28B, 28C, and 28D, herein collectively referred to as sub-receiver module 28, in digital section 23. Each sub-receiver 28 processes the raw digitized samples in order, inter alia, to generate initial five-level values for a subsequent decoder 30, common to all sub-receivers. Decoder 30 uses the five-level values to generate a combined output which is transferred to PCS 32, and from there to a Gigabit Media Independent Interface (GMII).
Each sub-receiver 28 also generates information for controlling attenuation levels of hybrid circuit 31 (in the corresponding transmitter module 40) and VGA 25 (in the corresponding receiver module 26). In addition, each sub-receiver 28 generates information which is used within a digital signal processing (DSP) management block 36 for setting a phase of PLL 38, when transceiver 20 acts as a slave, as is described in more detail below.
A fractionally spaced interpolator (FSI) 52 receives two samples per symbol, within an 8 ns period, from its respective FIFO 50. The FSI interpolates the two samples, and outputs one interpolated result at a phase derived from phase control block 66. The FSI also provides fine alignment to further correct skew occurring between pairs of conductors. By interpolating the samples, the interpolator improves the signal to noise ratio (SNR) of the signals by 3 dB, and also eliminates clock frequency noise.
Processing block 90 operates on samples x((n−0.5)×TS), x((n−1)×TS) and x((n−1.5)×TS) to produce an output fse_o2(n×TS) given by equation (1):
fse—o2=c·x((n−1.5)×TS)+x((n−1)×TS)+c·x((n−0.5)×TS) (1)
Processing block 92 operates on samples x(n×TS), x((n−0.5)×TS), and x((n−1)×TS) to produce an output fse_o1(n×TS) given by equation (2):
fse—o1=c·x((n−1)×TS)+x((n−0.5)×TS)+(1−c)·x(n×TS) (2)
A linear interpolation of the four samples is produced by adding fse_o1(n×TS) and fse_o2(n×TS) in a summer 94 to produce an output fse_out(n×TS) given by equation (3):
Outputs fse_o1(n×TS) and fse_o2(n×TS) are used as control inputs for a timing sensors block 62 (
The interpolation provided by FSI 52 uses a relatively simple system of interpolation wherein the interpolator, in addition to providing interpolation, acts as a low-pass filter and removes clock frequency noise completely. The filtration provided by FSI 52, when taken with a later adaptive equalizer of transceiver 20, substantially completely equalizes the channel it is operating on. It will be appreciated that the simple implementation of FSI 52, taken together with the later equalizer, provides a complete solution for equalizing the channel.
The control input configures equalizer 54 to operate as a blind equalizer when the equalizer is beginning to receive communications from remote transceiver 33, i.e., after the initialization phase (described above) between transceiver 20 and transceiver 33 has completed, and during a start-up phase of transceiver 20. The control input configures equalizer 54 to operate as an FFE when conditions in the blind equalizer configuration have stabilized, whereupon transceiver 20 enters an operational phase. Equalizer 54 also comprises a blind error producer 182, which generates a blind error value when the equalizer operates as a blind equalizer.
Initially, when transceiver 20 is not receiving a signal, i.e., when remote transmitters which would normally provide the transceiver with a signal are inactive, all tap coefficients of equalizer 54 are set to zero, apart from fourth tap coefficient c4(n), which is set equal to 4. Setting all tap coefficients to zero, apart from setting the fourth tap coefficient to 4, enables the filter to operate substantially transparently without performing equalization. Thus, coefficient c3(n) is set to a value 0 and is applied via the “0” path of a multiplexer 206 to a multiplier 210. Also, the “0” path of a multiplexer 218 is activated, so that a summer 214 is used.
Multiplier 236 outputs its value to a summer 238, a register 240 which provides a time delay and which feeds back to the summer, and a fixed point transformation (FPT) converter 242 to give a final coefficient output ci(n+1):
ci(n+1)=ci(n)+μtype·err(n)·ξ(n) (4)
where i is a coefficient index and n is a time index;
μtype is μblind or μffe, according to the operational state of the equalizer;
err(n) is the blind or the FFE error signal; and
ξ(n) is the signal value ν(n), after time n, derived from time delays such as delay 208 or 212 (
Returning to
y(n)=ξ(n)·c3(n)+[ν(n−1)−ν(n)]·c4(n) (5)
where the terms on the right side of equation (5) correspond to the output from multipliers 210 and 216 respectively.
In producer 182 a blind error, err(n), is determined by subtracting a threshold THLD value from |y(n)| (|y(n)| is generated in device 200) in a summer 202:
err(n)=|y(n)|−THLD (6)
The value of THLD is set according to whether the particular sub-receiver 28 comprising equalizer 54 operates as a master or as a slave. If the sub-receiver operates as a master THLD is set to be approximately 1.8. If the sub-receiver operates as a slave, THLD is set to be approximately 1.7.
Coefficient c3(n) is updated using equation (4). Coefficient c4(n) is modified using “differential” adaptation based on a value of (ν(n−1)−ν(n)), as shown in equation (5).
Equalizer 54 continues to operate as a blind equalizer until coefficients generated by the equalizer have converged to approximately constant values. At this point the equalizer is converted to an FFE equalizer by allowing paths “1” in multiplexer 206 and 218 to be followed, and by using the following transformations at the time of change-over:
ffe—coeff3(n)=c3(n)−c4(n), ffe—coeff4(n)=c4(n), ffe—coeffi(n)=0, i≠3, 4. (7)
Returning to
Equalizer 54, when operating as an FFE equalizer, removes all the inter-symbol interference (ISI) caused by the symbol transmitted two cycles ago, i.e., the symbol prior to an immediately previous symbol. This allows each sub-receiver 28 to have a DFE without a second tap, i.e., having a second coefficient set effectively to zero, as is described in more detail below with respect to
Single tap 59 comprises a summer 244 and a multiplier 246. Multiplier 246 receives a hard decision from slicer 58 and a first coefficient C1, and their product is input to summer 244 after a delay of a single clock cycle. Summer 244 also receives the delayed output of tail DFE 60, via a register 61, as described below. The summer's output is used as an input to summer 57.
DFE 60 comprises ten substantially similar taps, a third tap to a twelfth tap, the third and fourth taps receiving respective preliminary decisions P3, P4, from a Viterbi decoder in decoder 30, the fifth and sixth taps receiving a preliminary decision P5, and the seventh to twelfth taps receiving a preliminary decision P6. Each tap comprises a summer 248, and a multiplier 250 which also receives a coefficient C3, . . . , C12. The output of tail DFE 60 is provided, via a register 61 providing a time delay, to summer 56 wherein it is subtracted. The delayed output of DFE 60 is also provided to single tap DFE 59. A second tap 63 of combined DFE 65 has a coefficient set to zero, so that the second tap of the combined DFE comprises substantially only a time delay, with no coefficient multiplication.
Block 252 also receives the hard decision, herein termed hdec, produced by slicer 58. As described below, block 252 alters a value of coefficients Cn if |hdec| is 2; if |hdec| is not 2, Cn is unaltered. In comparators 262 and 264 hdec is evaluated and outputs of the comparators feed an OR gate 266. Gate 266 outputs 1 if |hdec|=2, and 0 if |hdec|≠2. The output cyin of comparator 262, checking if hdec is −2, is also input as a select signal to multiplexer 256. The output muxout of multiplexer 256 is err_μ_n if hdec is −2, otherwise muxout is err_μ.
A summer 258 receives outputs from multiplexer 256, comparator 262, and a time delay 260. Delay 260 receives the output of gate 266, and is enabled if the output is 1. The output of block 252 is thus given by equations (8a) and (8b):
Ci+1=Ci+muxout+cyin (|hdec|=2) (8a)
Ci+1=Ci(|hdec|≠2) (8b)
The five parallel first DFE taps 280A, 280B, 280C, 280D, and 280E of decoder 30 for each channel of the decoder, while receiving five-level data from two cycles, need to predict only five possible levels of an existing symbol, rather than 25 combinations as is described in prior art systems such as that of U.S. Pat. No. 6,192,072, to Azadet, referred to in the Background of the Invention, so that the complexity of the MDFEs is correspondingly reduced.
In energy sensor 310 inputs fseo2 and fseo1 are respectively filtered in filters 316 and 318, and an absolute value of each filtered output is generated in devices 320 and 322. Filters 316 and 318 are implemented to emphasize their inputs, and preferably have a transfer function given by equation (9):
where z−1 represents a delay of one clock cycle.
A summer 324 calculates the difference between the two absolute outputs, the difference is filtered in a leakage filter 325, and transferred to a multiplexer 314. It will be appreciated that the difference (between the filtered values of fseo2 and fseo1) provides an error signal which is zero when fseo2 and fseo1 are equal.
Coefficients sensor 312 performs the operation given by equation (10):
Δtcoeff(n)=ffe—coeff3(n)−dfe—coeff1(n)−TO (10)
where Δtcoeff(n) is the timing error;
As stated above, each sub-receiver may operate as a master or as a slave. As a master, TO is assigned to be between approximately −4 and −3. As a slave, once the coefficients of the equalizers have converged, TO is assigned to be equal to (ffe_coeff3(n)−dfe_coeff1(n)).
The output Δtcoeff(n) is transferred to multiplexer 314. Multiplexer 314 selects between the two timing errors, from energy sensor 310 and coefficients sensor 312, depending on a state of operation of sub-receiver 28. If the sub-receiver is operating as a master only, the output from the coefficients sensor is used. If the sub-receiver operates as a slave, and there is no transmission from the corresponding transmitter 40, the output from the energy sensors is used, since there is substantially no echo noise. After the sub-receiver that is operating as a slave starts to transmit, the output from the coefficients sensor is used. The facility to switch between energy sensor 310 and coefficients sensor 312 significantly improves the robustness of operation of transceiver 20.
The preliminary value of c is input to a summer 444, which also receives a delayed value of c from a calculation block 446 so as to provide integration of c. The summed result from summer 444 is output to a c calculation block 446, which outputs the value of c, and an increment (+1) or decrement (−1) signal which is sent to FIFO 50, according to table I below.
TABLE I
Initial c value
Increment/Decrement
c value output
c > 1
−1
0
c < 0
+1
1
0 ≦ c ≦ 1
0
c
c is then transferred to FSI 52, where it is used as described above with reference to
Immediately after the initialization phase, multiplexer 466 selects and outputs the timing error from selector 460. Once the receiver has stabilized, i.e., the coefficients of each sub-receiver 28 have converged to approximately stable values, multiplexer 466 selects and outputs the averaged value of all the timing errors. The output of multiplexer 466 is transferred via a loop filter 482 to an increment/decrement control block 484, which receives the increment/decrement control provided to FIFO 50. Responsive to the multiplexer output, block 484 generates an increment or a decrement signal to alter the phase of PLL 38.
Returning to
The error is then right shifted by μ, each μ preferably being set within a range from 9 to 16 depending on an adaptation step size desired. The values of μ are most preferably pre-set at implementation of receiver 28. How the values of μ are utilized is described in more detail below. In addition to the signals described above, control logic 500 receives other control inputs, also described below, for operation of cancellers 70 and 72. A summer 502, comprising registers, receives outputs from cancellers 70 and 72, and the summed output is input as described above to summer 56. As described below, echo canceller 70 also measures a round trip delay (RTD) of signals transmitted from a corresponding transmitter 40, and outputs a flag RTD_done which may be used by receiver 28 to determine if the receiver is in its initialization state.
Echo canceller 70 and NEXT cancellers 72 are implemented from adaptive finite impulse response (FIR) filters.
Each section of adaptation pipeline 512 computes a coefficient of the form:
Cn+1m=Cnm+errn·xn·2−μ (10)
where
The output of equalization pipeline 514, i.e., the value output by the leftmost adder 528, is given by
yn=Cn0·xn+Cn−11·xn−1+Cn−22·xn−2+Cn−33·xn−3 (11)
where xn−p is a value of xn delayed by p cycles.
yn=Cn0·xn+Cn−11·xn−1+Cn−12·xn−2+Cn−23·xn−3 (12)
The hybrid form of FIR architecture has a number of advantages, known in the art, compared to the transpose-form architecture illustrated in
Multipliers 524 in adaptive FIR filters of preferred embodiments of the present invention most preferably use ones complement multiplication in each tap of the filter. Using ones complement arithmetic reduces both area and power requirements compared to implementing twos complement arithmetic. An accumulator is used to adjust the final result, as shown in the following derivation.
Equations (11) and (12) may be represented by:
which may be separated into positive Xn+ and negative Xn− values to give:
When Xn is negative, this can be rewritten:
which can be further rewritten to:
where ˜Cn in the second term in equation (13d) is the ones complement of coefficient Cn, f is a number of fractional bits in Cn, and the last term in equation (13d) is the adjustment needed for using ones complement arithmetic.
Filter 650 further comprises elements which act as an accumulator 660 for adjusting the overall result from the filter, enabling the filter to implement equation (13d). Accumulator 660 comprises selectors 659 that operate generally as selectors 654 receiving Xn and Xn−M, a summer 661 that sums the output of selectors 659, and a register 663 that inserts a delay into the output of the summer. Accumulator 660 is implemented to produce the adjustment corresponding to the last term of equation (13d), enabling filter 650 to transform back from ones complement to twos complement arithmetic. When an input Xn is negative its absolute value is added to accumulator 660; when data Xn−M leaving a last tap of the filter is negative, its absolute value is subtracted from the accumulator. The adjustment is added to the filter output in an adjustment-adder 529, which produces a final filter output. Those skilled in the art will be able to formulate a schematic, generally similar to that of
Echo canceller 72 and NEXT cancellers 70 preferably comprise hybrid-form or transpose-form FIR filters generally similar to filter 650. As described in more detail below with reference to
As stated above, input data to NEXT cancellers 70 and Echo canceller 72 are five-level signals {+2, +1, 0, −1, −2}. Some preferred embodiments of the present invention perform adaptation when values of the input are 2 or −2, and not for the other three values. In the initialization phase of receiver 28, inputs to the cancellers are also set to be +2, 0, or −2, so that during this phase there is no effect on the adaptation. After completion of the initialization phase, performing adaptation using only the +2 and −2 levels leads to significant savings of chip area and chip power consumption with minimal reduction in adaptation performance. The minimal reduction is due to the fact that most of the energy of the input values resides in the +2 and −2 levels.
During the initialization phase of receiver 28, filter coefficients of NEXT cancellers 70 and Echo canceller 72 are most preferably adapted without decimation. Once the initialization phase has concluded, however, the coefficients typically tend to vary relatively slowly, so that adaptation decimation may be implemented with relatively little loss of efficiency. Preferred embodiments of the present invention preferably implement adaptation decimation in a substantially random manner over time, so as to minimize any frequency dependent adaptation factor. Most preferably, the randomness is introduced by cyclically counting the number of +2 or −2 values on the input data. Each time the counter returns to a specific value, e.g., 0, the next +2 or −2 value is used for adaptation. The value is used one tap at a time, i.e., the value “traverses” the adaptation pipeline so that at a time tn it is used for tap 1, at a time tn+1 it is used for tap 2, and continues until the last tap. Most preferably, the adaptation rate generated by the counter is monitored, and in the event of the adaptation rate falling below a minimum frequency, the counter is overridden and adaptation is enforced for the next +2 or −2 input value, so ensuring a minimum adaptation rate.
In transferring the five levels {+2, +1, 0, −1, −2} between elements of the cancellers, the levels are encoded as binary strings. Toggling between the binary values because of level changes, as data is transferred, uses power. Some preferred embodiments of the present invention use an encoding scheme for the levels and strings defined by the following one-one relationship: {(level, string)}={(+2, 010), (+1, 001), (0, 000), (−1, 100), (+1, 110)}.
The encoding is a mapping between the two parameters level and string, and is also shown in Table II below.
TABLE II
Level
String
+2
010
+1
001
0
000
−1
100
−2
110
Analysis of the encoding of Table II shows that a total of 32 transitions occurs for toggling between any level value to any other level value. The total of the transitions forms a measure of a switching activity value that would be caused by toggling between the levels in a random manner, as is typically the case when data is transferred. A prior art encoding scheme encodes −1 as 111, other encoding values being as given in Table II. Using the encoding scheme of Table II leads to a significant saving in numbers of transitions needed for toggling between any two data levels, and thus to a reduction in switching activity value, compared to the prior art scheme. The reduction in switching activity value leads to a corresponding reduction in power used.
Equalization region 701 receives its raw data as one of five levels, {−2, −1, 0, +1, +2}, and translates the data in a translation block 705 as described above with reference to Table II. The translated data is input to an equalization pipeline block 702, which comprises a number of delays according to the hybrid factor selected, so that block 702 is followed by an equalization select block 710, wherein appropriately delayed input data is applied to the corresponding filter taps according to the hybrid factor.
Data from block 710 is input to an equalization block 712, wherein the data is multiplied by filter coefficients, derived from the adaptation region, using ones complement arithmetic, as described above. As also described above, the ones complement arithmetic requires an adjustment Yadj, which is performed by an accumulator comprised in an equalization adjustment block 708, substantially as described above with reference to
An adder-register block 706 comprises adders and registers of the output path of the filter. The number of adders and registers is a function of the hybrid factor of the filter, and adders are preferably combined where possible. For example, a hybrid factor of 7 (23−1) requires, in the output path, seven consecutive adders between every two registers. Each seven adders may be combined into one 8-adder with eight inputs—one input from each respective filter tap plus one input from a neighboring register holding a previous partial result. The adder-register block 706 outputs the final result Yout of filter 700, before adjustment by the output of equalization adjustment block 708. The two outputs may be combined to give a final output of filter 700, or, for example when filter 700 is cascaded with other similar filters, the two outputs may be kept separate, as described in more detail with reference to
Adaptation region 703 also receives its input data as one of five levels, {−2, −1, 0, +1, +2}, and translates the data in a translation block 707, substantially similar to block 705, and the two blocks may be effectively combined to one block. The translated data is filtered through a 3-level adaptation block 709, which only transfers {−2, 0, +2}, so that region 703 performs adaptation on levels +2 and −2, as described above. The filtered values are received by an adaptation decimation block 711, which decimates the adaptation rate by a predetermined value, as described above.
The decimated adaptation data is input to an adaptation pipeline block 704, which comprises a delay for each tap of structure 700. The delayed adaptation data from block 704 is multiplied, in an adjustment block 713, by the error signal shifted by a pre-determined parameter μ, and the resultant product is output to update coefficient values of structure 700, and also as an input to equalization block 712.
An RTD controller 758, a delay line 752, and two RTD filters 754 and 756 implement parameters of the delay line and the RTD filters to correct the round trip delay. Delay line 752 preferably comprises two equivalent delay lines, one for the equalization data in filters 754 and 756, and a second for the adaptation data in the filters. Each delay line may preferably introduce a delay of up to 124 cycles, and the delay is most preferably set in steps of 4 cycles. Each of the delay lines in delay line 752 is preferably implemented from separately powered registers and delays comprised in the respective delay lines.
RTD controller 758 adjusts respective delays introduced into the input data by delay line 752, by setting two indices tap_index1 and tap_index2. The respective delays are used to position each of RTD filters 754 and 756, each comprising 16 taps, in the overall filter so as to absorb maximum echo energy. A preferred method of positioning the RTD filters is described below with reference to
Filter blocks 780, 782, and 784 have an effective hybrid factor of 7, since the output of block 784 is the summation of the its seven taps. and since the five taps of block 780 are combined with its previous result, with the outputs of RTD filters 754 and 756 in summers 760 (
Filter block 786 and RTD filters 754 and 756 have a hybrid factor of 16, and so utilize a 16-input adder. These filters most preferably generate a 20-bit coefficient with a dynamic range between + 1/4 and − 1/4 for internal operation, and the nine MSBs of the coefficient are used for equalization. Reducing the number of MSBs allows optimal use of the 16-input adder.
As shown in
Blocks 780, 782, 784 and 786 may perform adaptation decimation, as described above with reference to structure 700, according to predetermined values set by decimation setting parameters adp_dec input to control logic 500. The decimation rate for blocks 780 and 782 can preferably be set at a value of 2, 4, or 8. The decimation rate for blocks 784 and 786, and for filters 754 and 756 (
Dividing the canceller 72 into two tap regions—a “lower tap” region comprising blocks 780 and 782 with 33 taps, and a “higher tap” region comprising blocks 784, 786, 754, and 756 with 55 taps—allows for more flexibility in setting decimation values. The flexibility enables power to be saved, by having high decimation values, without performance degradation. Preferably, the lower taps are set to have low decimation values, and the higher taps are set to have high decimation values.
In an initial state 802, before controller 758 activates, block 754 is set to be directly after the last tap of filter 750, and block 756 is set to be directly after the last tap of block 754. Thus, if block 786 in filter 750 is activated, blocks 754 and 756 have effective tap positions 57–72 and 73–88; if block 786 is not activated, blocks 754 and 756 have effective tap positions 41–56 and 57–72.
In an activation step 804, controller 758 activates during the initialization phase of receiver 28 and waits a pre-determined time for the taps of the active blocks in filter 750 to converge.
At a beginning of a recursion sequence step 806, controller 758 resets the coefficients of blocks 754 and 756, and allows the taps of these blocks to converge.
In an energy measurement step 808, the absolute values of the tap coefficients for each block 754 and 756 are summed. The two sums are used as a measure of the energy being input to the respective blocks.
In an index calculation step 810 controller 758 adjusts initial values of tap_index1 and tap_index2, the indices respectively governing the delays for RTD blocks 754 and 756. The indices are preferably adjusted in steps of 4 or 8 taps. After adjustment, the block with the lowest energy, as measured in step 808, moves to its new index, unless one of conditions 812 and 814 is true, as shown in a step 815.
In a check overlap condition 812, the positions of the taps of blocks 754 and 756 are checked. If there is no overlap between the blocks, process 800 continues to a check out-of-bounds condition 814. If there is overlap between the blocks, the process continues to an overlap-exists step 816.
In check out-of-bounds condition 814, controller 758 checks to see that the values of tap_index1 and tap_index2 are within a predetermined bound. If the values are within the bound, process 800 returns to step 806. If one of the indices exceed the bound, process 800 stops.
In overlap-exists step 816, rather than the block with the lowest energy moving, as in step 810, the block with the highest energy moves to the new index value. Controller resets the block taps and waits for the taps of the block in its new position to converge.
In a check-energy condition 818, controller 758 then checks if the energy of the block in the new index value is higher than the block's previous energy. If the energy is higher, then the block remains at the new index value. If the energy is not higher, then the block reverts to its previous index value. Process 800 then continues by returning to a position after step 808.
Once process 800 has completed, RTD blocks 754 and 756 are in positions having highest measured energies. It will be appreciated that the process enables blocks comprising 32 taps to be adaptively moved so that maximum echo energy is absorbed. It will be further appreciated that using blocks having adjustable positions saves considerable power compared to static systems, such as full echo cancellers known in the art, which require considerably more taps to absorb the maximum echo energy. Preferably, controller 758 is implemented to power down one of the RTD blocks if its measured energy is smaller by a predetermined factor than the other RTD block, further reducing power consumption of the filter. The predetermined factor is set by an rtd_pwrdn parameter input to control logic 500. Further preferably, controller 758 is preferably implemented to power down unused registers in delay line 752, further reducing power consumption of the filter.
Filter block 850 preferably comprises 14 filter taps; filter block 852 preferably comprises 28 filter taps, and both filters are implemented with a hybrid factor of 7, and each adder in the output path is most preferably an 8-adder. In addition, the last 7 or the last 14 taps in filter block 852 may most preferably by powered down, according to a predetermined factor next-pwrdn input to control logic 500, so that the overall number of taps in canceller 70 may be set to be 28, 35, or 42.
Most preferably, filter block 850 maintains a 19-bit coefficient with a dynamic range between
and
for internal operation, and filter block maintains a 17-bit coefficient with the same resolution as those of block 852, but with a dynamic range between
and
for internal operation. Preferably, for equalization block 850 uses the eight MSBs of its coefficients, but block 852 uses the six MSBs of its coefficients. Both filters generate fixed point results, block 850 having a resultant dynamic range between +2 and −2, block 852 having a resultant dynamic range between
and
Input data, Din, are cascaded from block 850 to block 852. The outputs Yout and Yadj, the latter most preferably comprising a ones complement adjustment as described above, are cascaded from block 852 to block 850. A summer 854 sums Yout and Yadj to produce the final Yout from canceller 70.
Most preferably, each NEXT canceller 70 is implemented to perform adaptation decimation according to one or more predetermined adaptation decimation parameters input to the canceller. The decimation is preferably implemented to be at a rate chosen from the values {4, 8, 16}; alternatively, no decimation may be implemented. In addition, canceller 70 is preferably implemented to have an adaptation rate monitor, as described above, which activates if the measured decimation rate falls below a predetermined value, preferably four times an effective decimation rate. Decimation and rate monitoring are preferably implemented by methods generally similar, mutatis mutandis, to those described above with reference to
The final Yout outputs from each NEXT canceller 70, as shown in
It will be appreciated that the scope of the present invention is not limited to a specific number of transmission lines acting as channels of communication, and that the number of lines may be substantially any plurality of lines.
Process 900 is most preferably implemented by transceiver 20 during the initialization stage (described above with reference to
Transceiver 33 generates idle symbols and scrambles the idle symbols, according to a predetermined polynomial defined in standard 802.3ab, so as to generate scrambled idle symbols SIA, SIB, SIC, SID. The idle symbols generated are a function of a mode of communication between transceiver 33 and transceiver 20, which is determined by the values of transceiver 33 parameters loc_rcvr_status and mod—2. loc_rcvr_status defines a status of the overall link as being satisfactory or not. mod_2 defines a type of idle symbol to be transmitted. mod_2 and loc_rcvr_status may each take a value of 0 or 1.
SIB and SID are dependent on mod_2, and so for a specific data-symbol there are two possible symbols SIB and two possible symbols SID. SIC is dependent on loc_rcvr_status and mod—2, and so for a specific character there are four possible symbols SIC, SIA, SIB, SIC, SID are, by way of example, assumed to be transmitted at levels 2, 0, or −2, although it will be understood that they may be transmitted at substantially any plurality of levels.
In a translation step 902, processor 37 translates each level of SIA, SIB, SIC, SID to a string of three binary bits, most preferably according to Table II above.
In a storage step 904, processor 37 stores one of the binary bits of each string, most preferably the central bit, in respective cells of FIFO 39. FIFO 39 preferably comprises a reference column 39A (
In a synchronization step 906, processor 37 and de-scrambling code stored in storage memory 43 preferably operate as data de-scrambler 47. Alternatively, de-scrambler 47 is implemented by a substantially hardware system. Processor 37 synchronizes the de-scrambler to the scrambler of remote transceiver 33, using the data stored in column 39A, by methods which will be apparent to those skilled in the art.
In a generate idle codes step 908, processor 37 inputs the first bit in FIFO column 39A to de-scrambler 47, which calculates the four possible idle codes (corresponding to the possible values of loc_rcvr_status and mod_2) and the corresponding possible values for channel B (2 values), channel C (4 values), and channel D (2 values). De-scrambler 47 and processor 37 thus act as a symbol predictor, generating the four possible idle codes as expected values of channels B, C, and D.
In a comparison step 910, processor 37 compares the calculated expected idle codes with corresponding columns 39B, 39C, 39D in FIFO 39. The results of the comparison are stored in elimination memory array 45. Array 45 has a depth of (2n+1), corresponding to the depth of FIFO 39, and comprises 8 columns 45B0, 45B1, 45C00, 45C01, 45C10, 45C11, 45D0, 45D1, each column corresponding to a possible type of channel symbol, as described above. Thus, for the two possible idle codes generated for channel B, processor 37 compares each of the 2n+1 bits stored in column 39B with the calculated idle bits. The processor performs a similar process for each of the 2n+1 bits stored in column 39C (comparison with four idle codes) and for each of the 2n+1 bits stored in column 39D (comparison with two idle codes).
In an elimination step 912, processor 37 marks each of the cells in array 45 where the comparison indicates no match.
Processor 37 then advances the bits in FIFO 39 by one cell, in a continuation step 914, and repeats steps 908, 910, and 912. In step 910, however, the processor only compares bits in FIFO 39 which still show as being matched (after step 912).
Steps 908, 910, 912, and 914 are repeated until only a single match is present in one of columns 45B0, 45B1, one of columns 45C00, 45C01, 45C10, 45C11, and one of columns 45D0, 45D1, after which process 900 stops. It will be appreciated that at this point, the vertical displacement of each of the single matches corresponds to the skew of each of channels B, C, and D relative to channel A. In addition, the columns of array 45 having the single matches indicate the values of mod—2 and loc_rcvr_status, so indicating the mode of communication between transceiver 20 and transceiver 33.
It will be appreciated that the above description applies to three non-reference channels, each of the channels possibly being in more than one state depending on a communication mode of transceiver 33 and transceiver 20, and that the number of columns of memory array 45 corresponds to the total number of combined channels and states. It will thus be appreciated that increasing the number of columns of array 45 allows process 900 to identify skew values and channel states in correspondingly greater numbers of channels and/or larger numbers of possible states of the channels.
At an initial time 924, columns 39A and 39B of FIFO 39 have been filled by incoming bits, as described above for steps 902 and 904. Bits in respective cells of column 39A are differentiated by letters a, b, c, d, e, f, and g. De-scrambler 47 then synchronizes to channel A, and identifies bit a as the bit to be operated on, de-scrambling bit a to generate 1—corresponding to the “1” directly above broken line 922. These operations correspond to steps 906 and 908.
In operations corresponding to steps 910 and 912 processor 37 compares the 1 generated by the de-scrambler with bits in FIFO column 39B to determine bits which match the 1, and stores the matches and “no matches” in array column 45B. Matched bits are shown as √, non-matched bits as x. After time 924 there are seven matched symbols in column 45B.
At a time 926, bits in FIFO 39 are advanced by one cell and the process described above for time 924 repeats. In this case, the de-scrambler identifies b as the bit to be de-scrambled, and generates from bit b the value 0—corresponding to the 0 directly above broken line 922. The 0 is compared with bits in column 39B where array column 45B indicates there is still a match. As shown in
At a time 928, symbols in FIFO 39 are again advanced by one cell and the process repeats. The de-scrambler generates from symbol c the value 1, and this is compared with the four bits in column 39B where array column 45B indicates there is still a match. After this comparison there is one remaining match in array column 45B, and the process ends.
As is illustrated in
It will be appreciated that on a statistical basis, each comparison of processor 37 eliminates approximately half of the remaining matched symbols, until the single matched symbol corresponding to the skew remains. Furthermore, processor 37 is able to perform the comparisons for the different channels, and for the different possible idle codes for each channel, substantially in parallel. Thus, relatively few cycles of processor 37 are required to completely determine the skew of all incoming channels as well as the states of the different channels.
It will be understood that the principles described above may be applied to determining the skew between any multiplicity of channels upon which a signal is transmitted, by storing the skewed signals in a memory and sequentially comparing values using an elimination array until only one matched value remains in the array. It will be appreciated that the signals may be scrambled or non-scrambled. Furthermore, it will be appreciated that FIFO 39 and array 45 only need to have binary cells, regardless of the number of levels comprised in the incoming symbols. Alternatively, FIFO 39 may comprise cells which are capable of storing symbols having more than two levels.
The system described above with reference to
It will thus be appreciated that the preferred embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.
Greiss, Israel, Taich, Dimitry, Bublil, Baruch, Jacob, Jeffrey
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